Various adsorbents have been previously developed for use in fields including deodorization or exhaust gas treatment. Active charcoal is a typical example thereof. Active charcoal has been widely used in various industries in applications including air purification, desulfurization, denitrification or removal of harmful substances by utilizing its superior adsorption performance. More recently, due to the growing demand for nitrogen gas for use in applications including semiconductor production processes, a method for producing nitrogen is thus used that comprises producing nitrogen from air by pressure swing adsorption or temperature swing adsorption using molecule sieve carbon. In addition, molecular sieve carbon is also applied to the purification and separation of various gases, such as the purification of hydrogen from methanol cracking gas.
When separating a mixed gas by pressure swing adsorption or temperature swing adsorption, molecular sieve carbon or zeolite and the like is typically used as a separating adsorbent, and separation is carried out by the differences in its equilibrium adsorption amounts and/or adsorption rates. However, in the case of separating a mixed gas by the difference in equilibrium adsorption amounts, since the conventional adsorbents are unable to selectively adsorb only a gas that is desired to be removed, the separation factor is smaller, making increased size of the apparatus unavoidable. In addition, in the case of separating a mixed gas by the difference in adsorption rates, although only a gas desired to be removed can be adsorbed depending on the type of gas, since it is necessary to alternate between adsorption and desorption, the apparatus inevitably becomes large in this case as well.
1,3-butadiene is an example of a hydrocarbon gas that is targeted for separation and recovery. 1,3-butadiene is a compound that is useful as a starting material for the production of synthetic rubber as well as an intermediate of an extremely large number of compounds. 1,3-butadiene is typically produced by thermal decomposition of naphtha or dehydrogenation of butene. In these production methods, 1,3-butadiene is obtained in the form of one component of a mixed gas. Thus, it is necessary to selectively separate and recover 1,3-butadiene from this mixed gas. Examples of main components in the mixed gas having four carbon atoms include 1,3-butadiene, isobutene, 1-butene, trans-2-butene, cis-2-butene, n-butane and isobutane. Since these compounds have the same number of carbon atoms and similar boiling points, they are difficult to separate from each other using industrial distillation methods.
Another example of a separation method is extractive distillation. Since this method is an absorption method that uses a polar solvent, such as DMF, an extremely large amount of energy is required when recovering 1,3-butadiene from the polar solvent. Thus, an adsorption method to separate and recover 1,3-butadiene using less energy is desired.
However, since conventional porous materials (Patent Document 1) exhibit low separation performance with respect to the target gas, multi-step separation is required, thereby leading to unavoidable increases in size of the separation apparatus.
Porous metal complexes that induce a dynamic structural change by an external stimulus have been developed as adsorbents that provide separation performance superior to that of conventional porous materials (Non-Patent Document 1 and Non-Patent Document 2). In the case of using the porous materials described in these publications as gas adsorbents, a unique phenomenon has been observed in which, although gas is not adsorbed below a certain pressure, gas begins to be adsorbed once that pressure is exceeded. In addition, a phenomenon has been observed in which the gate-opening pressure varies depending on the type of gas.
In the case of applying this porous material to an adsorbent in a pressure swing adsorption system, gas can be separated extremely efficiently. In addition, the range of pressure swing can be narrowed, thereby contributing to energy savings. Moreover, this can contribute to downsize and cost-reduction of the gas separation apparatus, enabling to enhance cost competitiveness for both high-purity gas products and finished products made from the high-purity gases.
However, to meet growing demands for even greater cost reductions, it is necessary to further improve adsorption and separation performance.
A metal complex [Zn(R-ip)(L)] (wherein R represents H, CH3, NO2, Br or I, and L represents 1,2-di(4-pyridyl)ethylene, 1,2-di(4-pyridyl)ethane or 4,4′-azopyridine) has been disclosed and the complex is composed of various types of isophthalic acid derivatives, zinc ion and a dipyridyl compound capable of bidentate coordination with a metal ion (Patent Document 2 and Non-Patent Documents 3 to 6). However, these disclosures do not mention the effect of mixing two or more types of bidentate dipyridyl compounds on adsorption and separation performance.
Metal complexes composed of a zinc ion, various types of isophthalic acid derivatives and 4,4′-bipyridyl have been disclosed during the course of studies about the impact of mixing ligands on gate-opening pressure (Patent Document 3 and Non-Patent Document 7). However, these disclosures also do not mention the effect of mixing bidentate dipyridyl compounds on adsorption and separation performance.